In this case, the first indefinite term (with fourth derivatives) in
(REF ) is small, and the sign of the energy is defined by the
second term, providing a relevant constraint on the action
(REF ). The positivity of the theory in this limit does
not depend on fourth time derivatives. However, the kinetic energy
can be still unbounded from below for the negative coupling
\(\lambda < 0\) . Owing to
the violated dispersion relation between the wave vector \({{\bf k}}\) 
and the time derivatives, it is possible to have a large \({{\bf k}}^2\) 
with the conditions (REF ) satisfied. Thus, the sign of the
coupling \(\lambda \)  in the action (REF ) should be positive,
as it was always assumed in the literature, e.g., in the classical
works [1]}, [2]} and [3]}.
